The technology review industry really loves measurements. A new graphics card is tested on multiple resolutions on a ton of different games, most often with different graphics settings as well. Then it's subjected to a set of other measurements such as power consumption and noise, after which the measurements are demonstrated on a jungle of bars and graphs. Most other hardware components are subject to a similar set of tests and measurements.

Not mice however. The reviews generally have at least a few pictures of the mouse and often also of the box it came in. A list of promised features are shown, such as "3.5G Sensor". The review also has pictures of individual buttons and shapes and then a series of screenshots covering every tab on the bundled software. If there are weights or leds or other oddities those are usually very well covered.

As for the actual functionality of the mouse, most review sites state that performance of a mouse is an personal opinion, then they proceed to test the mouse through a very scientific "we gamed for a couple of hours" -method and then state that the gaming mouse xxx is a good gaming mouse.

Here are some examples:
"There are not any real benchmarks for mouse performance nor is there a conclusive way of determining which mouse performs best at a given setting. We used the mouse in a variety of applications from Photoshop to Battlefield 2142 without an issue. In general, gaming was extremely solid and we did not encounter any issues to speak of after a couple of weeks of testing. Overall, the vast majority of gamers will throughly enjoy this mouse."Anandtech

"Mice and other peripherals fall into an interesting category for testing as your opinion on them tends to be very personal, as the mouse or keyboard has to be right for you, beyond having the specifications to do what you want with it. Therefore these reviews can be highly subjective so as always, please make sure to test a mouse for yourself before you buy it.
However with that said, our testing methods involve using the mouse over prolonged gaming and general usage periods to test it's gaming ability, functionality and comfort."xreviews

"To test the Razer DeathAdder, Logitech New G5, and the Logitech MX518, I’m going to be testing for speed, comfort, and precision. You may notice that I did these tests already for the MX518 and the new G5, but these tests will be different because I’m using the Razer Mantis “Control” Mouse Pad. This test is not used in conjunction with any games; just me moving them on the mouse pad in different directions and speeds. All of the tests are out of 10; 10 being perfect and 1 being terrible."
overclockersclub He did not explain where the 1-10 number comes from. I assume from the top of his head; he then proceeded to make bar charts of the 3 numbers.

Part of these arguments are certainly valid. The shape of the mouse either fits your hand or it does not. The buttons are at good positions for your fingers or they are hard to press or easy to press by mistake. That's because every hand is different.

The most important part of the mouse is the sensor. That is a piece of hardware which CAN be tested. If you go searching there must be a ton of different mechanical devices that produce precise movement. Hook the mouse to that device somehow, or alternatively, move a surface under the mouse. Much like how fans are tested; you don't just state that it's loud, you use a decibel meter.

These are possible tests I came up with:
If we move the mouse in a repeated circle at a steady speed, does the cursor also move in a circle? Does it move on an oval instead? Does it eventually wonder off?

If we make the same sweeping motion multiple times, does the cursor make the same path every time?

Can the sensor actually do the acceleration the box promises?

How is the acceleration of the cursor related to the acceleration of the mouse? How long is the distance traveled by cursor on screen in relation to the acceleration of the mouse when moved a certain distance on a pad?

Does it work evenly on multiple surfaces? Does it work on Roccat Taito -mousepad? Does it work on Ulti-mat Breathe? Does it work on Ulti-mat classic? How about Icemat? How about gaming mousepad brand xxx? How about an uneven surface such as a wooden table?

Can the cursor be affected by holding down some/all buttons while moving the mouse?

There are more tests than I can come up with on the spot. One of the things you certainly should measure is consistency; if you do the same thing twice do you get the same result?

I'm surprised how content the review sites are stating that mice can't be tested and then just leaving it at that. At least try; get a 2nd mousepad if nothing else.